reduce allocations when handling crypto data

[marten-seemann]

Closes #4614. Part of #3663.

name          old time/op    new time/op    delta
Handshake-16     612µs ± 2%     609µs ± 3%    ~     (p=0.172 n=14+15)

name          old alloc/op   new alloc/op   delta
Handshake-16     244kB ± 0%     243kB ± 0%  -0.43%  (p=0.000 n=13+14)

name          old allocs/op  new allocs/op  delta
Handshake-16     2.50k ± 0%     2.50k ± 0%  -0.16%  (p=0.002 n=15+15)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.19%. Comparing base (b8ea5c7) to head (263beb6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4615      +/-   ##
==========================================
+ Coverage   85.18%   85.19%   +0.01%     
==========================================
  Files         151      151              
  Lines       14764    14757       -7     
==========================================
- Hits        12576    12571       -5     
+ Misses       1679     1678       -1     
+ Partials      509      508       -1     

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